PROJECT SUMMARY Adenosine deaminase acting on RNA (ADAR) converts adenosine to inosine specifically in dsRNA (A- to-I RNA editing). Our pioneering studies demonstrated the involvement of A-to-I RNA editing in the control of miRNA biogenesis and function. We recently found that loading of the EBV (Epstein-Barr-virus) miRNA miR- BART6 onto a functional RISC complex is inhibited by A-to-I editing of its primary transcript (pri-miR-BART6) by ADAR1. Moreover, four binding sites of miR-BART6-5p were identified within the human Dicer mRNA 3'UTR, revealing a unique strategy of EBV to manipulate the host RNAi mechanism. Tumor metastasis is the most common cause of death in patients with cancer, including breast cancer. The Epithelial-to-Mesenchymal Transition (EMT) plays an important role in metastasis. Snail, Slug, and Twist have been identified as major regulators of EMT. In addition to these conventional transcription factor proteins, miRNAs have emerged as new key factors that control EMT. Recent studies suggest that two miRNAs, miR- 103 and miR-107, promote the EMT by silencing Dicer and thereby repressing global synthesis of miRNAs, including a major EMT-inhibitory miRNA, miR-200. EBV is one of the most common human viruses, infecting more than 90% of the world's population, and association of latent EBV infection with a variety of human cancers such as Burkitt's lymphoma, Hodgkin's disease, and nasopharyngeal carcinoma is well established. EBV is frequently detected in human breast cancer specimens. Furthermore, more frequent detection of EBV in higher grade (metastatic) breast cancers has been reported. However, very little is known about the relevance of EBV to breast cancer causation and in particular metastasis. Recent studies by us indicate a previously unexplored possibility that EBV infection may play an important role in progression of breast tumors to metastasis. We hypothesize that miR-BART6-5p and its editing by ADAR1 control EMT through targeting Dicer, and contribute to metastasis of EBV positive breast cancer. Information obtained through this research proposal may lead to a new intervention for the prevention or therapeutic treatment of metastatic breast cancer. The long-term goal of this project is to better understand functions of EBV miR-BART6 in metastasis and the control mechanism mediated via A-to-I RNA editing. Specifically, we will determine: 1) the function of miR-BART6 RNAs in promotion of EMT in human breast cancer cell lines; 2) the significance of RNA editing in the EMT promoting function of miR-BART6; 3) the role of miR-BART6 in vivo in breast cancer metastasis using an orthotopic tumor implantation mouse model; and 4) the miR-BART6 and ADAR1 expression levels in human breast tumor specimens and their relevance to metastatic progression.